This disclosure relates generally to electromagnetic (EM) telemetry within a borehole, and more particularly to telemetry that provides particular capabilities for providing communication between sensors permanently deployed outside casing in a borehole.
During completion and servicing of a borehole, sensors can be employed along the exterior of the borehole casing to monitor the condition of well-servicing and other fluids in the annulus formed between the casing and borehole wall, or, more generally, to monitor conditions of the wellbore, of the surrounding formation, and/or of tools and materials used inside the wellbore. For example, when a borehole is completed by securing the casing in place with cement or another suitable sealant (such as, e.g., a polymer, resin, or latex-based sealant) (all such sealants generically referred to herein as “cement”), it is often desirable to measure one or more cement parameters (e.g., resistivity, permittivity, moisture content, temperature, pH, and/or ion concentration) in order to monitor the integrity thereof. Fluid-sensing approaches used for this and similar purposes include, without limitation, RFID tag detection and EM-based fluid characterization.
In various implementations, the sensors are deployed outside (or “behind”) the borehole casing, for instance, mounted to the exterior surface of the casing. A system for transmitting signals from these sensors to the surface for processing and evaluation is needed. In many circumstances, however, signal transmission via electrical or optical cables does not pose a viable solution for downhole-to-surface telemetry. For offshore wells, for example, regulations may require that no cables be clamped to the exterior of the casing because any such cables could present a potential conduit for fluid flow that could result in failure of the well's integrity. Accordingly, systems and methods for wireless telemetry between sensors deployed downhole and a surface-based control and data-acquisition/processing facility are needed.